Why ventilation commissioning at the end of the job is too late

On most residential projects, commissioning sits near the bottom of the programme. Ceilings closed, decorators finished, keys nearly handed over, and then somebody arrives with an instrument to record what the ventilation is actually doing. If the numbers are right, everyone signs. If they are not, you are looking at opening a plasterboard ceiling in a flat that has already been snagged.

I have spent years supervising ventilation installation on apartment blocks and townhouses, and the pattern rarely changes. The problem is not that commissioning is done badly. It is that commissioning is the wrong place to find out.

What actually happens when commissioning is left to the end?

Commissioning is a measurement, not a repair. The engineer records what the system delivers and compares it with what the design says it should deliver. That is the whole job. They cannot re-route a duct, cut a new hole, move a unit or replace a crushed length of flexible.

So when a flat fails, the fix belongs to somebody else, weeks or months after that somebody left site. What could have been corrected in ten minutes during first fix becomes a return visit, an access arrangement, protection to finished floors and making good afterwards.

And it rarely fails alone. If one flat of a type fails, the same detail is usually repeated across the level, and often across the block, because it came from the same drawing and the same sequence.

Why do the failures show up so late?

Because almost every cause is buried by the time anyone measures. In practice the recurring ones are:

• Flexible duct crushed or over-bent behind a bulkhead or above a ceiling.
• A run extended or diverted around structure that was not on the drawing.
• A hole cut in the wrong position, or not cut at all, with the route improvised to suit.
• The wrong plenum or terminal type fitted because the schedule and the drawing disagreed.
• Joints not properly sealed, so the system leaks into the ceiling void instead of outside.
• A unit position moved on site to clear another trade, lengthening the run.

None of those are visible once the ceiling is boarded and skimmed. The person taking the reading at handover is measuring the sum of every decision made months earlier, with no way of seeing which one caused the number in front of them.

Which problems are genuinely impossible to fix at handover?

Anything to do with route. If the duct is inside the structure or above a finished ceiling, you have three realistic outcomes: open up completed work, accept a system that underperforms, or argue about who pays. All three are expensive and the third one also costs you the relationship with the main contractor.

It is worth being fair about the other side of this. Some things genuinely belong at the end. Setting the unit, balancing, final adjustment at the terminal, filters, the handover pack. Those are commissioning tasks and they should stay there. The point is not to commission early. It is to stop discovering installation problems at commissioning.

What does checking in stages actually look like?

Four points, none of which takes longer than a walk round with a phone camera.

Before first fix. Drawing against the actual plot. Are the openings where the drawing says, does the route clear the structure and the other services, does the unit position exist in reality. This is where a clash costs nothing.

During first fix, before anything is boarded. Route, supports, bends, joints sealed, no crushed flexible, unit position matching the drawing, terminals the type the schedule calls for.

Before ceilings close. The last honest chance to see anything. One photo per run, per plot reference.

At commissioning. Now the measurement is a confirmation rather than a discovery, and when something is queried you have a dated record of what was installed instead of a memory of it.

The detail that makes this work is the plot reference. Photos filed by plot, not by date or by whoever took them, is the difference between evidence and a folder nobody opens.

Does staged checking not just slow the job down?

It adds time at first fix and removes considerably more at the end. A check on an open ceiling costs minutes. A return visit to a finished flat costs access, protection, making good, a second mobilisation and a hole in your programme, and it usually lands during the weeks when everyone is trying to hand over.

There is a commercial side to it too. If an opening was formed in the wrong place by another trade, a dated photograph taken at first fix is the difference between an agreed variation and a free re-do. That is the same principle as keeping your RAMS, accreditations and insurance visible: the evidence is only worth anything if it was captured at the time and can be found later.

Where does software actually help with this?

Not with the physics. Sizing, design intent and the required performance figures are engineering decisions, and no app changes them. If you want a quick sanity check on a run at design stage, our duct sizing tool does that part.

Where software earns its place is the record. Most ventilation contractors already collect the right information and then lose it: photos in a WhatsApp group, notes in a site diary, a tracker on somebody's laptop, and a snagging list rebuilt from scratch every time the main contractor asks. A simple per plot checklist on a phone, with photos attached to the plot and a status you can filter, turns the same effort into something you can hand over, defend and reuse on the next block.

Where to start

Do not redesign your QA process. Add one stage to the next block: a photo check per plot before ceilings close, filed by plot reference. Then compare return visits on that block against the last one. If there are fewer, the case makes itself and you can add the earlier stages.

If you want a second opinion on how your ventilation capability comes across to main contractors before they ever ring you, our free website audit looks at what your site actually says about your QA, evidence and handover process. If the answer is that you need the process itself tightened rather than the website, our ventilation work is where we would pick it up.